TEHRAN — Iran and six world powers have agreed on how to implement a nuclear deal struck in November, with its terms starting from Jan. 20, officials announced Sunday.

The announcement, made first by Iranian officials and later confirmed elsewhere, starts a six-month clock for a final deal to be struck over the Islamic Republic’s contested nuclear program. It also signals an easing of the financial sanctions crippling Iran’s economy, though some U.S. lawmakers have called for tough measures against the country.

Iran’s official IRNA news agency quoted Iranian Deputy Foreign Minister Abbas Araghchi confirming the news. The agency said Iran will allow the United Nations’ atomic agency access to its nuclear facilities and its centrifuge production lines to confirm it is complying with terms of the deal.

European Union negotiator Catherine Ashton praised the deal in a statement, saying “the foundations for a coherent, robust and smooth implementation … have been laid.” German Foreign Minister Frank-Walter Steinmeier called the deal “a decisive step forward which we can build on.”

U.S. Secretary of State John Kerry welcomed the deal in a statement as well, saying further negotiations “represent the best chance we have to resolve this critical national security issue peacefully, and durably.”

Under the November agreement, Iran agreed to limit its uranium enrichment to 5 percent — the grade commonly used to power reactors. The deal also commits Iran to stop producing 20 percent enriched uranium — which is only a technical step away from weapons-grade material — and to neutralize its 20 percent stockpile.

In exchange, economic sanctions Iran faces would be eased for a period of six months. During that time, the world powers — Britain, China, France, Germany, Russia and the United States — would continue negotiations with Iran on a permanent deal.

The West fears Iran’s nuclear program could allow it to build a nuclear bomb. Iran says its program is for peaceful purposes, such as medical research and power generation. Iran’s semi-official ISNA news agency reported Sunday that under the terms of the deal, Iran will guarantee that it won’t try to attain nuclear arms “under any circumstance.”

In a statement, U.S. President Barack Obama said the deal “will advance our goal of preventing Iran from obtaining a nuclear weapon.”

“I have no illusions about how hard it will be to achieve this objective, but for the sake of our national security and the peace and security of the world, now is the time to give diplomacy a chance to succeed,” Obama said.

However, U.S. lawmakers are proposing to blacklist several Iranian industrial sectors and ban banks and companies around the world from the U.S. market if they help Iran export any more oil. The provisions would only take effect if Tehran violates the interim nuclear deal or lets it expire without a follow-up accord. However, that has caused anxiety in Iran, where hard-liners have already called the deal a “poison chalice” and are threatening legislation to increase uranium enrichment.

In his statement, Obama said “unprecedented sanctions and tough diplomacy helped to bring Iran to the negotiating table,” but cautioned against implementing any more.

“Imposing additional sanctions now will only risk derailing our efforts to resolve this issue peacefully, and I will veto any legislation enacting new sanctions during the negotiation,” he said.